A multiplexer (MUX) is a device used to transmit a multiplicity of signals to a single line. If there are several signal lines in a system and these signal lines are to be separately placed on a single line a MUX is a device which can, via a control signal, select which of the several signal lines is placed on the single line.
An example usage of MUX's is switching addresses to a Dynamic Random Access Memory (DRAM) device. Because DRAM's are generally arranged in a matrix, or array, configuration both row and column addresses are used to address a specific location within the DRAM array. However, it is common in the art to use the same signal lines for both row addresses and column addresses in order to reduce the total number of input/output pins connected to the DRAM device. What is common in the art is to multiplex the row and column addresses such that at one point in time the signal lines are used for row addresses and at another point in time the signal lines are used for column addresses.
It is thus important that the MUX used to select between the row addresses and the column addresses properly switch between them when so desired. An incorrect selection can, of course, result in an incorrect MUX output. Furthermore, when an incorrect MUX output is first made and then corrected shortly thereafter a voltage spike is created. Voltage spikes, in turn, create current spikes. Because ElectroMagnetic Interference (EMI) is directly related to the amount of current in electrical circuits, large current spikes create large amounts of EMI. It is undesirable to have excessive amounts of EMI because of governmental regulations which limit the amount of EMI allowable in electronic equipment. Therefore, not only do prior art MUXes temporarily provide incorrect outputs but they also cause undesirable EMI conditions.